Immunity to RecombinantPlasmodium falciparumMerozoite Surface Protein 1 (MSP1): Protection inAotus nancymaiMonkeys Strongly Correlates with Anti-MSP1 Antibody Titer and In Vitro Parasite-Inhibitory Activity

Abstract

A number of malarial blood-stage candidate vaccines are currently being tested in human clinical trials, but our understanding of the relationship between clinical immunity and data obtained from in vitro assays remains inadequate. An in vitro assay which could reliably predict protective immunity in vivo would facilitate vaccine development. Merozoite surface protein1 (MSP1) is a leading blood-stage malaria vaccine candidate, and anti-MSP1 antibodies from individuals that are clinically immune to malaria inhibit the invasion of Plasmodium merozoites into erythrocytes in vitro. Using expression in Escherichia coli and subsequent refolding, we have produced two allelic forms of MSP1 42 (FVO and 3D7). Aotus nancymai monkeys were immunized with MSP1 42 -FVO, MSP1 42 -3D7, or a combination of FVO and 3D7 allelic forms, (MSP1 42 -C1) and were subsequently challenged with Plasmodium falciparum FVO parasites. Sera obtained prior to challenge were tested by standardized enzyme-linked immunosorbent assay (ELISA) to determine antibody titer, and immunoglobulin G (IgG) fractions were also obtained from the same sera; the IgG fractions were tested in an in vitro growth inhibition (GI) assay to evaluate biological activity of the antibodies. Regardless of the immunogen used, all monkeys that had >200,000 ELISA units against MSP1 42 -FVO antigen before challenge controlled their infections. By contrast, all monkeys whose purified IgGs gave <60% inhibition activity in an in vitro GI assay with P. falciparum FVO required treatment for high parasitemia after challenge. There is a strong correlation between ELISA units (Spearman rank correlation of greater than 0.75) or GI activity (Spearman rank correlation of greater than 0.70) and protective immunity judged by various parameters (e.g., cumulative parasitemia or day of patency). These data indicate that, in this monkey model, the ELISA and GI assay values can significantly predict protective immunity induced by a blood-stage vaccine, and they support the use of these assays as part of evaluation of human clinical trials of MSP1-based vaccines.

Addition of CpG ODN to recombinant Pseudomonas aeruginosa ExoProtein A conjugates of AMA1 and Pfs25 greatly increases the number of responders

“…Many studies have shown that the ability of these antigens to inhibit or block the development of malarial parasite is antibody dependent [15,[17][18][19][20]. Thus, the induction of immune responses with high antibody titers is thought to be critical for both candidates to be effective vaccines in humans.…”

“…GIA was performed on total IgG purified from pooled immune sera [11,15]. The mouse sera from each immunization group were pooled and the total IgG purified.…”

Adjuvant formulations possess differing efficacy in the potentiation of antibody and cell mediated responses to a human malaria vaccine under selective immune genes knockout environment

“…We believe that this apparent paradox is likely due to animal species differences in response to adjuvants since in the rodent malaria MSP1 vaccine model system, strong protective immunity can be induced by other less toxic adjuvant formulations [70][71][72]. For the P. falciparum MSP1 vaccine antigen described here, protective immunity is associated not with the level of total antibody responses but with the ability to induce parasite inhibitory antibodies [66,73,74]. We have recently conducted a comprehensive analysis (Hui et al, submitted) on the ability of the nine adjuvant formulations to induce parasite inhibitory, anti-MSP1-19 antibodies in different cytokine and co-stimulatory molecule gene KO mice [75,76], including those described in this study.…”

Immunogenicity of the Merozoite Surface Protein-1 (Msp-1) of Human Plasmodium Sp.

“…A significantly lower parasitemic level was detected in the rhesus monkeys immunized with E. coli-expressed P. vivax MSP-1 42 compared to the negative control group upon a challenge with P. cynomolgi, a P. vivax-closely related Plasmodium sp., blood stage parasites (49,50). On the other hand, A. nancymai vaccinated with E. coli-expressed P. falciparum MSP-1 42 was highly protected during a lethal P. falciparum challenge (51)(52)(53), and the protective effect was stronger than the baculovirus-expressed P. falciparum MSP-1 42 (54). Moreover, specific antibodies and antigenspecific T-cell responses with the production of IFN-γ were also detected in M. mulatta which were immunized with DNA plasmid encoding P. falciparum MSP-1 42 (55).…”